The present invention is a process for preparation of cyclic organohydrosiloxanes. The process comprises contacting an organohydrodihalosilane with a substantially stoichiometric quantity of water to form a mixture containing cyclic and short-chain linear organohydrosiloxanes. This mixture is distilled to separate the cyclic and linear organohydrosiloxanes from other components of the mixture. The separated cyclic and linear organohydrosiloxanes are then contacted with an additional quantity of water to effect hydrolysis and condensation of the linear organohydrosiloxanes to higher molecular weight linear organohydrosiloxanes which are easily separated from the cyclic organohydrosiloxanes by a second distillation step.
The present process provides cyclic organohydrosiloxanes that are essentially free of linear organohydrosiloxanes. The cyclic organohydrosiloxanes formed by the present process are useful reactive intermediates that can be reacted with, for example, polydimethylsiloxanes to form block copolymers and with unsaturated polyethers to form surfactants, compatibilizing agents, and stabilizing agents. The cyclic organohydrosiloxanes formed by the present process are also useful as a cross-linker for other siloxanes to form silicone elastomers and gels.
Sauer et al., J. Am. Chem. Soc. 68:962 (1946). observed that when methyldichlorosilane was reacted with water a highly viscous oil was formed which within a few minutes was transformed into a rubbery gel. Sauer et al. teach that the formation of this high polymer can be avoided by running the hydrolysis process at low temperature in an ether solution. This process suffers both from the need for a low temperature to be maintained and from the use of a highly flammable solvent.
Curry, U.S. Pat. No. 3,484,468, issued Dec. 16, 1969, teaches making cyclic methylhydrosiloxanes by reacting a tertiary alcohol with an organohydrodihalosilane in the present of a solvent such as benzene. This process is less than optimal because it generates a chlorinated hydrocarbon as a by-product that has little value. Furthermore, the use of a tertiary alcohol as the source of oxygen is far more expensive than the water required in the present process.
Miller et al., U.S. Pat. No 3,714,213, issued Jan. 30, 1973, teaches a process for making cyclic organohydrosiloxanes by catalytically cracking and cyclizing linear polysiloxanes containing methyl substituents, hydrogen substituents, and high molecular weight chain terminal groups, such as hexyldimethysilyl groups. A major drawback to this process is the synthesis of the linear polysiloxanes with the high molecular weight chain terminal groups.
The present process provides for high yields of essentially pure cyclic organohydrosiloxanes without the use of low temperatures, solvents, catalysts, and chain terminators, as required by processes taught in the cited